1. Field of the Invention
The present invention relates to an ink jet printing method and an ink jet printing apparatus, and specifically, to a two-side printing in which printing is performed on both sides of a printing medium such as printing paper.
2. Description of the Related Art
One of the most important conditions to be considered in executing a two-side printing in ink jet printing is a show-through effect or a strike-through effect. In general, when a large amount of ink is ejected in printing images or the like, the ink may permeate through a printing paper to its side opposite to a side to which the ink has been ejected. As a result of this, an image may be observed in the opposite side or an image printed on the opposite side may be degraded.
To solve these problems, for example, the techniques described below have been disclosed. Japanese Patent Application Laid-open No. 5-032024(1993) describes a technique used for the two-side printing to change a dot pattern to reduce the density of dots to be printed. Japanese Patent Application Laid-open No. 7-314734(1995) describes a technique used for two-side printing to similarly change a dot pattern to reduce the dot density or to reduce driving power for a printing head and thus reduce the size of dots. By thus reducing the dot density or the dot size, the optical density of printed images can be reduced. As a result, the image printed on one side of a printing medium is less shown through the printing medium when viewed from another side of the printing medium because of its low optical density. Furthermore, Japanese Patent Application Laid-open No. 2000-059630 describes a technique used for two-side printing to change an under-color removal rate during a halftone processing so as to use a larger amount of black to reduce the total amount of ink used. This makes it possible to suppress the strike-through effect in which ink permeates through printing paper to the vicinity of the other side.
In recent years, it has been desirable that ink jet type printing apparatuses such as ink jet printers provide prints that are at the same level as that of silver salt photographs. One of the arrangements that can achieve this is the use of plural types of the same color ink (hereinafter simply referred to as “light and dark ink”) provided with different color development densities by varying the concentration of color materials such as dyes or varying the color materials themselves. This light and dark ink achieves, for example, a smooth variation in the gradation of a printed image. This contributes to improving the print grade. In particular, it has been found that the addition of light ink is effective in reducing the granularity of a natural image having a large number of halftones.
If for example, a natural image is printed using such light and dark ink, a large area of the image is often occupied by relatively bright parts such as flesh color, sky blue, and the like. In this case, in the prior art, for these bright portions, the amount of light ink landed is increased, while the amount of dark ink landed is reduced. Furthermore, to achieve the same density, the amount of light ink must be larger than the amount of dark ink.
Accordingly, if a two-side printing is executed using light and dark ink, a larger amount of ink is used in the bright parts. This is disadvantageous in terms of the show through effect or the like. In particular, the amount of solvent increases consistently with increasing of the amount of light ink used, and thus increases the possibility of an occurrence of the show through effect.
When a two-side printing is executed using such light and dark ink, the show through effect may be suppressed by applying the technique of reducing the dot density or dot size as described in Japanese Patent Application Laid-open No. 5-032024(1993) and Japanese Patent Application Laid-open No. 7-314734(1995). However, this technique may severely degrade image quality such as the gradation or granularity of the image. For example, if the dot density or dot size is reduced in bright parts of the image such as flesh color or sky blue, the optical density in these parts decreases. Consequently, the density becomes discontinuous between these parts and other parts. On the other hand, if the dot density or dot size is reduced all over the image, the entire optical density decreases. Furthermore, a decrease in dot density particularly causes dark dots to be dispersed to increase the granularity.
The results are similar if the technique described in Japanese Patent Application Laid-open No. 2000-059630 is applied. Although the total amount of ink used can be reduced, tints of the printed image may be markedly different from those in, for example, one-side printing owing to an increase in under-color removal rate.